1. Field of the Invention
The present invention relates to an ink droplet eject apparatus and method, and more specifically to an apparatus and method to eject a fine ink droplet suitable for graphic image printing on a printing medium.
2. Description of the Related Art
Various kinds of ink eject devices have been proposed, of which seven examples will described with reference to the accompanying drawings.
A first example shown in FIGS. 20(A) and 20(B) is an ink droplet eject apparatus for use in an ink jet printer or the like. As shown in FIG. 20(A), a piezoelectric element 101 is vibrated so as to expand the volume of an ink accommodating chamber 102. Liquid ink 103 is sucked from an ink tank (not shown). As shown in FIG. 20(B), the volume of the ink accommodating chamber 102 is then reduced so as to apply a pressure to the ink 103 therein. An ink droplet 103a is ejected from a nozzle 104 onto a printing medium such as a paper. This art is described in U.S. Pat. No. 3,946,398.
The invention described in Japanese Patent Publication No. 61-59911/1986 is similar to that in U.S Pat. No. 3,946,398. In the Japanese publication, a heating element is contained in the ink droplet eject chamber. Heat energy causes a bubble to be instantaneously generated in the ink. An expansion force of the bubble causes the ink droplet to be ejected.
In the second example shown in FIG. 21, an ink fountain 202 is formed in a glass plate 200. A piezoelectric element 201 is provided for drawing the ink 103 along a flow path 204 into the ink fountain 202. Between a silicon board 205 and the glass plate 200 is disposed a dry film 206 which forms a space and a plurality of flow paths or the like. An eject heater 207 is heated. The heat energy causes a bubble to be instantaneously generated in the ink. The ink is ejected. This art is described in Japanese Patent Application Laid-open No. 5-112008/1993.
The second example is characterized in that inks having different dyestuff densities are supplied from a plurality of flow paths to the ink fountain 202 thereby allowing a density gradation.
Thus, in the above-described prior art, various ink droplet eject apparatuses using a pumping principle have been proposed.
On the other hand, ink droplet eject apparatus ejecting the ink in a mist state are described, for example, in Japanese Patent Application Laid-open No. 4-14455/1992, Japanese Patent Application Laid-open No. 4-299148/1992, Japanese Patent Application Laid-open No. 5-38810/1993, Japanese Patent Application Laid-open No. 4-355145/1992, and Japanese Patent Application Laid-open No. 5-508/1993.
A third example is disclosed in the above-listed Japanese Patent Application Laid-open No. 4-14455/1992.
In the third example, as shown in FIGS. 22(A) and 22(B), plural pairs of tandem electrodes 304 are formed at one end on a propagating surface 301A of a piezoelectric propagating plate 301. The electrodes 304 are used as drive means so as to apply a high-frequency alternating voltage E of about 20 MHz. The propagating surface 301A is excited so as to generate a surface elastic wave (surface wave). In this drawing, numeral 302 denotes the ink fountain. Numeral 302A denotes an ink flow path.
The generated surface elastic wave progresses in a direction shown by an arrow A. The surface elastic wave reaches a portion where the propagating surface 301A is in contact with the liquid ink 103. At this time, the surface elastic wave is transmitted to the ink 103 so as to be a longitudinal elastic wave (ultrasonic wave). This elastic wave causes an ink surface 307 exposed by a slit 306 to be excited. The ink droplet 103a is ejected in the mist state.
The fourth example shown in FIG. 23 is disclosed in Japanese Patent Application Laid-open No. 5-508/1993. In FIG. 23, a gap is formed between a slit member 308 and a resonator 309. An ink fountain 305 is disposed in the gap. Numeral 306 denotes a piezoelectric actuator.
In the fourth example, a capillary action initially causes the ink 103 to be filled in the ink fountain 305. A resonance vibration is applied to the resonator 309 in a thickness direction. Its vibration energy is propagated to the ink 103. Finally, random surface waves are formed on an ink interface 103A of an ink eject port 310. Interference of the surface waves causes an ink particle to be ejected in the mist state in accordance with a vibration frequency of the resonator 309.
The fifth example shown in FIG. 24 is disclosed in Japanese Patent Application Laid-open No. 5-38810/1993. In the fifth example, a pair of electrodes 403A and 403B are formed on both surfaces of a piezoelectric substrate 401. A nozzle plate 405 is joined to the piezoelectric substrate 401 via a gap support member 404. Capillary action causes the liquid ink 103 to be filled in the gap space.
An intersection region 406 is formed by the electrodes 403A and 403B. To the intersection region 406 is applied a voltage displaced by a resonant frequency determined by the thickness of the piezoelectric substrate 401. The piezoelectric substrate 401 is resonated so as to generate the ultrasonic wave in the liquid ink 103.
The generated ultrasonic wave is propagated in the ink 103. The surface wave is generated on the ink surface 103A of the ink 103 filled in a nozzle 405A just over the intersection region 406. When an amplitude of the surface wave is larger than the constant amplitude, the ink droplet 103a is discharged in the mist state.
Furthermore, a similar technique is also disclosed in Japanese Patent Application Laid-open No. No. 4-14455/1992, Japanese Patent Application Laid-open No. 4-299148/1992, Japanese Patent Application Laid-open No. 4-355145/1992, Japanese Patent Application Laid-open No. 5-508/1993 and Japanese Patent Application Laid-open No. 5-38810/1993. However, their means for generating the surface wave on an ink liquid surface are different from one another.
In any apparatus using as its ejecting principle an ultrasonic wave wetting apparatus, the surface wave is generated at random on a free surface of a liquid. The interference of the surface wave causes the ink droplet 103a to be ejected in the mist state from many unspecified ejecting points.
In addition, a sixth example shown in FIG. 25 is an ink droplet eject apparatus using an acoustic pressure by an acoustic streaming. It is disclosed in Japanese Patent Application Laid-open No. 63-162253/1988.
In this technique, as shown in FIG. 25, the vibration of a piezoelectric transducer 501 causes an ultrasonic acoustic wave to be generated. The ultrasonic acoustic wave is focused on one point on the free surface 103A of the ink 103 by a concave spherical surface acoustic lens portion 502A formed on an end surface (upper end surface in FIG. 25) of an acoustic lens body 502. When the acoustic wave collides with the free surface 103A of the ink 103, a radiation pressure is generated. The generated radiation pressure causes the ink droplet 103a to be ejected separately from the free surface 103A of the ink 103.
A seventh example shown in FIGS. 26(A) and 26(B) is an ink droplet eject apparatus using acoustic pressure by an acoustic streaming, as is the case with the sixth example described above. The seventh example is disclosed in Japanese Patent Application Laid-open No. 6-218926/1994.
In the ink droplet eject apparatus shown in FIG. 25, a plurality of piezoelectric elements 601 are disposed in a matrix. A predetermined voltage is applied to a group of piezoelectric elements 601 in a portion where the ink droplet is to be ejected. A concavity 602 is formed in order to concentrate a pressure wave. A high frequency voltage is then applied to the concavity so as to vibrate the concavity. This causes the ink droplet 103a to be ejected. Numeral 604 denotes the ink surface. Numeral 605 denotes an ink film layer. Reference character A denotes a direction of ink flow. Numeral 606 denotes a case body.
On the other hand, in various printers including ink jet type printers, a precise pictorial image output requires the ability to produce a continuous smooth density gradation from a high light side to shadow side.
The following methods are conventional for obtaining such gradation printing ability in the ink jet type printers. In one method, an amount of ink droplet to be ejected is varied for each pixel so as to perform a density modulation. In an other method, one pixel is composed of a plurality of ink droplets finer than a pixel size so as to perform the density modulation in accordance with the number of ink droplets. In either method, in order to express a smooth gradation without a gradation jump, a technique which ejects ink droplets sufficiently finer than the pixel size is essential.
However, in the above-described conventional ink droplet eject apparatus, it is difficult to realize the continuous smooth density gradation printing characteristics for the following reasons.
In the first and second examples of FIGS. 20(A), 20(B), and 21, each apparatus operates by the pumping principle. Therefore, the diameter of a minimum ejectable ink droplet is substantially the same as that of a nozzle.
Thus it is impossible in practice to eject a fine ink droplet, for example, having 1/10th the diameter of the nozzle diameter.
Ejecting fine ink droplets with these ink droplet eject apparatuses requires a reduced nozzle diameter according to a desired ink droplet diameter. However, the reduction of the nozzle diameter easily causes a clogging of the nozzle. Thus, reliability and durability of the overall apparatus deteriorates. Therefore, in the above-described conventional ink eject apparatus, it has been very difficult to form a fine ink droplet having a diameter of, for example, a few .mu.m to 20 .mu.m.
Moreover, the reduction of the nozzle diameter requires high precision manufacturing techniques. Therefore, in the ink droplet eject apparatus using the pump principle, besides the clogging of the nozzle described above, there is an inconvenience that productivity is low.
Furthermore, in the second example shown in FIG. 21, inks having different dyestuff densities can be ejected from one nozzle so as to perform a density graduation. However, it is difficult to uniformly mix the ink 103 and to continue ejecting it with a good reproducibility. Moreover, since an eject heater 207 and a piezoelectric element 201 are necessary, the cost of the ink droplet eject apparatus is increased.
Furthermore, in the third example, a surface wave is generated on a free surface of the liquid so that the ink droplet is ejected in a mist state. Also in the ink droplet eject apparatus described in Japanese Patent Application Laid-open No. 4-299148/1992, Japanese Patent Application Laid-open No. 4-355145/1992, and the above fourth and fifth examples, a fine ink droplet having a diameter of about a few .mu.m can be ejected in the mist state. Changing the eject time makes it possible to control the number of ink droplets to be ejected.
However, these ink droplet eject apparatuses adopt a construction wherein the interference is caused by surface waves generated at random on the free surface of the liquid. Therefore, ink droplets are ejected in the mist state from many unspecified ink droplet eject points. This creates a variation in the diameter of the ink droplet to be ejected. Moreover, the ejecting direction and ejecting velocity also vary for each ink droplet.
Thus, there is a disadvantage in controllability per ink droplet necessary for an ink jet printing head. That is, it is difficult to control a shot position of the ink droplet on the printing medium and an ink amount of the shot with high accuracy and stability.
Moreover, in the sixth example, the availability of vibration energy is low. Thus, a piezoelectric transducer 501 of FIG. 25 generating each ink droplet 103a is required to be large in size. Disadvantageously, this requires the size of the apparatus to be increased.
Furthermore, since a depth of focus of an acoustic lens is very shallow, means for realizing a high-accuracy control of the free ink surface position is necessary. Moreover, the acoustic lens is necessary for each ultrasonic vibrator. Disadvantageously, this causes the apparatus construction to be complicated.
In the seventh example, although the size of each piezoelectric element is small, a group of many piezoelectric elements 601 is necessary. A predetermined voltage is applied to each of the piezoelectric elements 601. Therefore, the construction is complicated, thereby disadvantageously resulting in the rise of the cost of the ink droplet eject apparatus.
Furthermore, an electric system needs a circuit construction having a band for passing a signal of a few hundreds of MHz such as a high frequency power amplifying/generating portion for performing an oscillation and an amplification of a high frequency signal from a few MHz to a few hundreds of MHz and a high frequency power switch portion for printing. Disadvantageously, this also increases the cost of the apparatus.